1. Field of the Invention
The present invention is directed to apparatus and methods for increasing the thermal efficiency of high temperature combustion sources such as glass and other process furnaces. More particularly, although not exclusively, the present invention is directed to methods and apparatus for enhancing fuel efficiencies of regenerative furnaces and as well as for reducing the fuel penalty that is associated with the use of gas reburning for NO.sub.X control with respect to the same.
2. Technology Review
The combustion of fossil fuels, especially coals and heavy oils, produces a significant amount of NO.sub.X which ultimately participates in the formation of smog and acid rain. This problem is exacerbated in the case of certain combustion sources such as glass furnaces, cement and ceramic kilns, and steel reheat furnaces which operate at extremely high temperatures. Such high temperature combustion sources produce even greater levels of NO.sub.X.
Currently, increasingly stringent air quality regulations impose requirements on glass manufacturing and other industries to reduce such NO.sub.X emissions. A number of technologies to reduce NO.sub.X emissions have been developed in response to such air quality regulations. A brief discussion of these technologies may be helpful to an understanding of the nature of the present invention.
Two technologies that have achieved generally moderate NO.sub.X reductions of approximately 40% to 50% at relatively low cost are low NO.sub.X burners and oxygen enriched air staging. These technologies offer good cost effectiveness (e.g., $400/ton of abated NO.sub.X), but don't always meet stringent air quality regulations. These technologies also exhibit a small fuel penalty, typically on the order of 2% to 5%.
Two other technologies, that are capable of achieving 85% to 90% reductions in NO.sub.X, are selective catalytic reduction and oxy-fuels. These technologies, however, suffer from high capital costs and high operating costs, respectively. Oxy-fuels exhibit poor cost effectiveness (typically $2500 per ton of abated NO.sub.X). Further, selective catalytic reduction is poorly suited to glass furnaces because the alkalis, that are present in the exhaust gas, can rapidly poison the catalyst.
Two other NO.sub.X reduction technologies, that have been effective in achieving up to 60% NO.sub.X reductions, are combustion modification techniques such as staged combustion and reburning. Gas reburning can be applied to high temperature combustion sources such as glass furnaces and can achieve NO.sub.X reduction levels similar to oxy-fuel, but with a cost effectiveness similar to low NO.sub.X burners and oxygen enriched air staging.
Reburning is a controlled process that uses fuel to reduce oxides of nitrogen that are collectively referred to as NO.sub.X. These oxides include NO (nitric oxide), NO.sub.2 (nitrogen dioxide), and N.sub.2 O.sub.4 (dinitrogen tetroxide), and N.sub.2 O (nitrous oxide). In the reburning process, a fraction of the total fuel, typically an amount of fuel yielding between 10% to 20% of the total heat input, is injected above the main heat release zone to produce an oxygen deficient reburning zone. The combustion of reburning fuel forms hydrocarbon radicals which react with nitric oxide to form molecular nitrogen, thus reducing NO.sub.X. This process occurs best in the at very low oxygen. Subsequently, burnout air is injected downstream of the reburn zone to combust the remaining fuel fragments and convert the exiting HCN and NH.sub.3 species to either NO or NO.sub.2.
Previous studies have shown that 60% to 80% reduction in NO.sub.X emissions can be achieved with natural gas reburning and that most of the reduction occurs in the reburning zone. NO.sub.X reduction in the burnout zone, via the HCN and NH.sub.3 species from the reburning zone, is minimal because of the high burnout temperature (2200.degree.-2400.degree. F.) and the presence of an excessive amount of carbon monoxide ("CO", above 2% at 0.9 stoichiometry).
The overall reburning process can be divided conceptually into three zones as follows:
Primary Zone: The primary or main heat release zone normally accounts for approximately 80 percent of the total heat input to the system and is operated under fuel lean conditions. The level of NO.sub.X exiting this zone is defined to be the input to 7 the reburning process. If sufficient residence time is not provided, unburned fuel fragments may leave this zone and enter the reburning zone. PA1 Reburning Zone: The reburning fuel is injected downstream of the primary zone to create a fuel rich, NO.sub.X reduction zone. Reactive nitrogen enters this zone from two sources: the primary NO.sub.X created in the main heat release zone and the fuel nitrogen, if any, in the reburn fuel. These reactive nitrogen species react with the hydrocarbon fragments formed during the partial oxidation of the reburning fuel, primarily CH species, to produce intermediate species such as HCN and NH.sub.3. Additionally, some nitrogen is converted to N.sub.2 and some is retained as NO. If the reburning fuel is a solid such as coal, nitrogen may also leave this zone as char nitrogen. PA1 Burnout Zone: In this final zone, air is added to produce overall lean conditions and to oxidize all of the remaining fuel fragments. The total fixed nitrogen species (TFN=NH.sub.3 +HCN+NO+char nitrogen) will be either oxidized to NO.sub.X or reduced to molecular nitrogen. PA1 (a) co-firing a heavy fuel oil or solid carbonaceous fuel with the primary fuel in the primary combustion zone such that the co-fired fuel is cracked and thereby generates a high number density of carbonaceous particles sufficient to improve soot radiation; PA1 (b) introducing the reburn fuel into the reburn zone at or near sonic speeds; PA1 (c) enhancing the mixing of first burnout air with the flue gas in a first burnout zone; and PA1 (d) promoting instability of boundary layer gases adjacent to the refractory packing. PA1 (a) mixing a reburning fuel with combustion emissions in a gaseous reburning zone such that the reburning zone is substantially oxygen deficient; PA1 (b) passing the resulting mixture of reburning fuel and combustion emissions into a first burnout zone; PA1 (c) introducing burnout air into the first burnout zone such that the carbon monoxide concentration in the first burnout zone is reduced to a level below about 500 ppm; PA1 (d) enhancing the mixing of first burnout air with the flue gas in a first burnout zone; PA1 (e) advancing the resulting mixture from the first burnout zone to a second burnout zone having a temperature in the range from about 1300.degree. F. to about 1900.degree. F.; and PA1 (f) introducing a second stream of burnout air into the second burnout zone, said second stream of burnout air including a reducing agent capable of providing a source of nitrogenous reducing species thereby reducing the nitrogen oxides in the combustion emissions. PA1 (a) mixing a reburning fuel with combustion emissions in a gaseous reburning zone such that the reburning zone is substantially oxygen deficient; PA1 (c) introducing burnout air into the first burnout zone such that the carbon monoxide concentration in the first burnout zone is reduced to a level below about 500 ppm; PA1 (d) advancing the resulting mixture from the first burnout zone to a second burnout zone having a temperature in the range from about 1300.degree. F. to about 1900.degree. F.; PA1 (e) introducing into the second burnout zone a reducing agent capable of providing a source of nitrogenous reducing species thereby reducing the nitrogen oxides in the combustion emissions; and PA1 (f) advancing the second stream of burnout air and the combustion emissions to a bed of refractory packing, the bed of refractory packing having a surface adapted to promote the tripping of the boundary layer gases proximal to it.
It will be understood, however, that the introduction of gas reburn fuel downstream of the primary combustion zone represents a potentially significant increase in operating costs over other available NO.sub.X reduction technology.
Another, more advanced technique for NO.sub.X reduction, which is capable of obtaining NO.sub.X reduction of up to 90%, is "advanced gas reburn." Advanced gas reburn technology represents a synergistic combination of gas reburn technology with selective non-catalytic reduction. The techniques for advanced gas reburn are set forth in U.S. Pat. No. 5,139,755 (hereinafter the "'755 patent") which is incorporated by reference herein. Advanced gas reburn provides a significant incremental improvement in NO.sub.X control over conventional gas reburn and other combustion modification techniques, with only a minimal fuel efficiency penalty (approximately $600 per ton of abated NO.sub.x) (which is similar to most other NO.sub.X reduction methods using combustion modification techniques).
The advanced gas reburn process has wide application to high temperature pyroprocessing and melting operations which employ direct fired combustion and furnace equipment. Such equipment is typically used in the manufacture of glass, refractory and steel products, and high temperature hydrocarbon (cracking) processes. Best results are typically obtained where post-combustion temperatures are in the range of 2200.degree. to 2900.degree. F.
In the case of glass furnaces, recovery regenerators or recuperators are commonly used to minimize thermal energy loss. Most large glass furnaces have staged regenerators where cavities exist. In the case of retro-fit advanced gas reburn, reburn fuel and burnout air are provided in a first cavity that is connected to the melting tank. A second or additional cavity, having a preferred temperature window of between 1650.degree. and 1900.degree. F., is provided downstream of the first cavity. The selective non-catalytic reducing agent, and burnout air, are introduced in the second cavity.
It will be understood, however, that the use of advanced gas reburn will nonetheless exhibit a potentially significant fuel penalty that is associated with the introduction of reburn fuel. While regenerators may be able to recapture 50% of this thermal energy loss, the remaining energy is lost. It will be therefore be understood that what is needed in the art are methods for optimizing reburning processes to thereby reduce the aforesaid fuel penalty to a minimum and to maintain the second cavity temperature in the preferred range.
It will also be understood that high temperature process furnaces and the like operate with substantial thermal inefficiencies. It will therefore be understood that it is desirable to minimize thermal inefficiencies that are associated with high temperature combustion sources. This would serve to further minimize the overall operating costs and hence reduce the aforesaid fuel penalty that is associated with reburning processes.
Such apparatus and methods are disclosed and claimed herein and comprise a number of control processes that may be used for optimization of gas reburn technology.